Oil exploration has moved into arctic waters. Motion of drifting ice is often a crucial problem when designing and planning an off-take loading and mooring system in ice infested waters. It is imperative to design systems and methods which eliminate the risks for pollution, caused by damage to the equipment due to impacts from the drifting ice.
The drifting motion of ice is mainly governed by wind, waves, ocean currents and tidal forces. From analyses for the Eastern Barents Sea, it has been found that on a large time scale the ice drifting motion is clearly stochastic and with the exception of periods with rather straight lined movement, it resembles Brownian motion. Since ice floes are generally large and heavy, the direction and absolute value of their speed cannot change momentarily. Models predict steady motion of the ice, but occasionally the direction of the ice drift may change to the opposite direction in roughly half an hour. This is a major concern for the conventional loading concept where the tanker, say 90 000DWT, is staying in the “wake” behind a platform or a tower extending up above the sea level. If using a sub sea loading concept instead in waters subjected to drifting ice, allowing the tanker to ‘ice-vane’, advantages may be achieved.
In ice-infested waters, however, bottom installations might be damaged by deep ice formations (ice ridges in the Pechora Sea, icebergs in some other places).
Tests executed in 1997 and 2000 at the Hamburg Ship Model Basin (HSVA), Germany, testing the Submerged Turret Loading system, STL, in frozen seas, showed that under-keel installations will be in contact with ice as soon as the ice conditions worsen (interactions with ice ridges). Hence, the riser has to be protected from this hazard.